The present invention relates to a metal gas cell, such as a nickel hydrogen battery, and more particularly, to a support assembly for a plate stack which is utilized in a metal gas cell.
Metal gas cells, in particular nickel hydrogen batteries, have evolved to include a plate stack encased within a sealed metal vessel. The vessel has a generally cylindrical configuration and is charged with a gas under pressure, such as hydrogen. Conventionally, a sheet of metal, such as a nickel alloy, is hydroformed into a hollow cylindrically shaped member having one hemispherically configured end portion by stretching the sheet around an appropriately shaped mandrel. Two such members are welded together to form a cylindrically shaped vessel or casing having two hemispherically configured end portions. The constraints of the hydroforming process limit the overall length of the vessel, for example a maximum length of approximately 14 inches can be obtained for a 3.5 inch diameter vessel.
As conventionally manufactured, the length of the pressure vessel, and thus the capacity of an individual cell, has been limited by constraints imposed by the process of hydroforming metal. Typical nickel hydrogen batteries which have a cell stack cantilevered from one
end thereof are typically 8-12 inches in length. In addition, the high tooling costs associated with hydroforming further constrains the vessel diameter which must be prescribed at the outset of the hydroforming process. Further, since the thickness of a hollow cylindrical shaped member which is hydroformed for construction of the pressure vessel varies and is smallest at the segment of the member intermediate the hemispherically configured end portion and the lip or edge of the member, the vessel must be designed to withstand a given burst of pressure using the minimum thickness thereby resulting in increased expense and vessel weight.
Components of a simple plate stack include a negative electrode plate, a gas diffusion separator, and a positive electrode plate. Each of these components is a relatively thin, e.g., 0.01-0.05 inch, disc or annular wafer having an opening or aperture through the center thereof. As assembled on an elongated core, the components are juxtaposed and aligned and are repeated in series to form the conventional plate stack of an individual metal gas cell.
Supporting a conventional plate stack within a sealed pressure vessel has previously been accomplished by inserting an elongated, generally cylindrical core having an end plate attached near one end thereof through the aligned apertures of the components of an assembled plate stack. A second end plate is releasably secured on the core in a contiguous relationship with the other end of the assembled plate stack. One end of the core is fixedly secured to the pressure vessel near one end thereof, usually by a weld ring which is utilized to weld the two members together which form the pressure vessel. Thus, the plate stack is cantilevered from one end of the core and the pressure vessel. Since a substantial portion of the pressure exerted by and weight of the plate stack is sustained by the elongated core, the core must be redesigned for each different size of cell in which it is employed so as to withstand the specific pressure and weight imparted to it. Further, since a nickel hydrogen battery functions as an energy source for satellites, the forces transmitted to the cell or battery during prelaunch testing, during launch and flight of the satellite dictate that the plate stack of an individual battery be supported so as to prevent damage from such forces. During satellite testing, launch, and movement in orbit, forces acting on a conventional battery can cause unsupported components of the plate stack, especially those components near the unsupported end of the core, to impinge upon the walls of the pressure vessel thereby damaging and/or shorting the electrodes. Such impingement can result in premature cell or battery failure.
It has been suggested to cantilever mount two separate plate stacks from a weld ring which is positioned approximately in the center of a pressure vessel to increase the energy capacity of a nickel hydrogen battery, as well as the integrity and durability thereof. However, an internal impedance to electrolyte communication between the two separate plate stacks is created by the centrally located weld ring. Additionally, each separate plate stack is cantilevered from a central support and thus may be subject to premature cell failure due to the forces encountered during launch and orbit of a satellite.
End plates used in metal gas cells have been constructed of two annular, correspondingly sized plates having an opening or aperture through the center thereof which are spaced apart and fixedly secured together by means of a plurality of ribbons. The plurality of ribbons are secured to each plate by any suitable means, such as by welding. The plates and ribbons are constructed of a nickel based alloy. However, conventional metal end plates are expensive and require a relatively long manufacturing lead time. And the relatively small rates necessary to produce end plates for the metal gas cell market do not justify the high injection mold costs associated with manufacturing plastic end plates. Thus, a need exists for an inexpensive, thin, e.g. 0.10 in., low volume, and light weight end plate for use in metal gas cells.
Accordingly, it is an object of the present invention to provide a metal gas cell wherein the plate stack which is enclosed within a pressure vessel is effectively supported along substantially the entire length thereof.
Another object of the present invention is to provide a support assembly for a plate stack utilized in a metal gas cell in which an elongated core on which the plate stack is positioned is not a substantial structural component for supporting the weight of pressure imparted by the plate stack.
It is also an object of the present invention is to construct a metal gas cell having a length which is not constrained by the process of manufacturing the pressure vessel component thereof.
It is a further object of the present invention to provide a pressure vessel for use in a metal gas cell which has a substantially uniform thickness, is lightweight, and can be manufactured to any given length.
It is still a further object of the present invention to provide an end plate for use in metal gas cells which is relatively thin, lightweight, and inexpensive and has a relatively low volume and high strength.